Not able to get enough current from buck boost (sepic) converter made using TL494

Question

I am trying to build a buckboost converter for converting 5-40v input to 5-35V output at 3A max to use it for many purposes by usign TL494. I am using a SEPIC topology as i want a non inverting output. I simulated the circuit in proteus. The circuit works fine when i connected a 500 Ohm load, I was able to get output of 5 to 35 volts at 100kHz frequency.

But when I used a 5 Ohm load, the voltage drops from 35v to 2v, because it can't able to give enough current, The maximum i get is somewhat around 400mA

I think the problem is in buck-boost stage, because I tested the buck-boost stage seperatly by giving a PWM signal, it has the same result.

I used inductors with 82uH and capacitors with 22uF, Shottkey diode and IRF540 MOSFET, I also change the shotkey diode with a high current one, but no results.

Here the simulation results,

with 500 Ohm load

with 5 Ohm load

waveform of L1 current, C1 voltage and PWM at 5 Ohm load

But, when i changed the capacitor values to 1000uF, inductor values to 220uH and frequency to 5kHz i get 2A for 10v, but still not 3A

1. So, what i need to do to get desired output, what are the parameters needed to tweak?
2. Do i have to operate the buck-boost stage at resonant frequency to get max output?
3. What limits it form providing high current?

Answer 1

Some considerations

1. The FB output can only sink 0.7 mA, so the value of R4 with 500 Ω is far too low. You just have 350 mV regulation head room. There is no need to use such a low impedance divider R4/R3. This is most probably the reason why the needed duty cycle of around 76 % is not reached.
2. The value of the gate discharge resistor R2 is too high for a switching frequency of 100 kHz. The gate capacitance to be discharged is around 1.5 nF.
3. IRF540 needs at least 7 V to turn fully on, so you need a TL494 supply of around 9 V minimum, with safety margin, to reach this (VCE = 1.5V). If you need a minimum input voltage of 5 V, you need another MOSFET with lower threshold voltage.
4. The 5 Ω load at 35 V gives 7 A output current, which is far above your specification.
5. The unconnected input pin 15 may be a problem, connect it to the 2.5 V reference.
6. Using 82 uH creates a relative low ripple current and needs physically large inductors at the required current of around 16A. Lower values have some advantages in this circuit.

With a matching duty cycle and proper gate drive the converter works in this simulation:

^{simulate this circuit – Schematic created using CircuitLab}